The present invention relates to methods for stirring molten metal in a continuous casting process, and, more particularly, where, in the continuous casting process, there is induced in a metal flow, defining a central casting axis, a moving electromagnetic induction field by means of an inductor arrangement disposed around the metal flow and supplied with a multiphase current, the field generating in the molten metal at least one movement transverse to the metal flow.
It is well known to carry out electromagnetic stirring in continuous casting in order to obtain greater regularity for the cast metal, both as regards its surface state and its internal properties, such as segregation and shrinkage.
It has thus been proposed to stir the metal either in the ingot mould itself, or at different places below the ingot mould. The exact point of positioning of the various inductors producing the stirring is determined as a function of the casting speed, the casting cross-section, and also of the quality of the metal to be treated.
Two main types of stirrers have hitherto been known:
a) the rotational stirrer which surrounds as symmetrically as possible the metal which is cast or is to be cast and acts perpendicularly to the flow of the metal; such a stirrer is for example described in the printed patent specification FR-A-2,279,500 which is incorporated herein by reference;
b) the directional linear stirrer which acts most often parallel to the flow of the metal; such a stirrer is for example described in the German journal "Fachberichte Huttenpraxis Metallverarbeitung", Vol. 25, No. 7, 1987, p. 676-681 "Electromagnetic Stirring using Voest-Alpine Pulsators on the Donawitz Continuous Bloom Caster" which is incorporated herein by reference. The latter type of stirrer may act either on only one surface of the cast metal, or on two diametrically opposed surfaces, or even on the four surfaces of the metal. It has also been proposed to use the latter type of linear inductor while applying pulses to it which are intended to generate an additional stirring of the metal to be treated.
The printed patent specification FR-A-2,485,411, which is incorporated herein by reference, and U.S. Pat. No. 4,867,786, which is also incorporated herein by reference, propose using linear inductors on two opposing faces of the metal. These inductors are then electrically divided into successive elementary inductive units producing juxtaposed rotational movements, rotating pairwise in opposing directions. The method relates essentially to metal products with an elongated quadrilateral cross-section.
It has also been proposed to generate in the metal to be treated, by means of rotational stirrers, a helicoid movement in order to entrain the inclusions contained in the liquid metal upwards. Such a method has for example been described in the printed patent specification FR-A-2,426,516 (corresponding to U.S. Pat. No. 4,281,263 the disclosure of both of which are incorporated herein by reference). This method has not however found an application because it hinders correct lubrication of the ingot mould.
Printed patent specification DE-A-3,527,387, which is incorporated herein by reference, proposes using two rotating magnetic fields, working at different frequencies and amplitudes. The objective sought is to create two different coaxial rotational movements wherein the external movement has a lower speed of rotation than the internal movement It will be noted that the axis of rotation of the two movements coincides with the central casting axis. In the same way as conventional rotational stirring, this method has, among the others, the disadvantage of creating a movement similar to a vortex at the center of the internal movement, when it is desired to intensify the stirring movements.
U.S. Pat. No. 4,877,079, which is incorporated herein by reference, proposes juxtapositioning, in the molten metal flow, two rotational movements of opposite rotation directions, so as to create, at the interface of the two movements, a material flow transverse to the central casting axis. The combined action of these movements consequently leads to better movement of the bath at the center of the metal flow. The main drawback of this method is the orientation of the movement along one dominant direction, which is certainly not optimal from the point of view of cross-sectional homogeneity of the structure of the metal.
It is consequently noted that, although it is true that the various techniques hitherto employed have contributed to an improvement of the internal and external structure of the continuously cast products, it is nonetheless true that the various types of stirring used lead to new non-negligible drawbacks. Thus it has, for example, been noticed that rotational and linear stirrers, used both in the ingot mould itself and beneath the ingot mould, can at a certain moment entrain in the metal to be treated movements promoting the creation of defects, for example: the formation of inclusions and white lines, or deterioration of the internal structure at the central segregation.